Genetic association of mating types and virulence for mice were found in Cryptococcus neoformans. A pair of congenic Cryptococcus neoformans var. neoformans strains, B-4476 (a) and B-4500 (alpha) that presumably only differ in mating type, was constructed. This pair and their progeny, five alpha type and five a type, were tested for virulence in mice. In the parent strains as well as the progeny, alpha type was clearly more virulent than a type. In addition, death tended to occur earlier among the mice infected with alpha strains than the mice that died by infection caused by a strains. All unstable URA 5 transformants (DNA mediated) of C. neoformans var. neoformans exhibited the presence of an autonomously replicating plasmid which had undergone significant sequence rearrangement. The autonomously replicating plasmids in the transformants were maintained in linear form and had acquired genomic sequences with homology to a sequence(s) on all the chromosomes. These results suggest mechanisms of plasmid maintenance in C. neoformans that are different from those reported for other yeasts.